Should I periodically air out the vapor barrier in the crawlspace

Assess the damage:if moderate, try removing the mold yourself. Use fans to move the air and flood lights to see and plastic covers to collect the mold when scrapped clean. Use liquid spray that you can buy at the hardware store.Some suggest surface blasters. After the mold area is clean and dry put in a dehumidifier and check for ventilation as preventative measures.

I am also in the Northeast and have had to tackle this type of situation a few times. There are several ways to attack it.

In your case, the best solution, but unfortunately the most expensive would be to use an open cell foam kit to fill the joist bays. They are fairly easy to use and give you about an R-4 rating per inch. They are a two tank kit with a hose and wand. Here is an example product: Touch N Seal 1000 Kit Open Cell Spray Foam Insulation

These type of kits start around $400, depending on the square footage and depth required.

A cheaper method would be to use 24 inch R-19 unfaced blanket insulation cut to your custom widths. You can use wire springs to hold it in place and keep it from falling. I would then cover the entire area with 4 to 6 mil plastic, stapled to all the joists. Tape the joints, if any. Moisture should not be a major problem since you have already done what sounds like a good job sealing the dirt floor. In the summer months, some ventilation in the crawl space will also help avoid excess moisture.

Rigid foam glued to the foundation walls would also help if the area is fairly air tight. If there is free air flow, insulating the brick walls would be a waste of time and money.